In fabricating electronic systems, integrated circuits ("chips") or other electrical devices are mounted on printed wiring boards, or other substrates. The contact between the chip and substrate must have physical, chemical and electrical integrity and stability. Copper is particularly useful as a layer in microelectronic devices, because of its good conductivity properties.
In one technique for physically and electrically connecting microelectronic devices, metal pads are fabricated on an exposed surface of a substrate. These metal pads are often formed with a top layer of solder, i.e., a low melting point alloy, usually of the lead-tin type, used for joining metals at temperatures about 230.degree. C. The solder pads are brought into contact with a metal structural element that will be wet with liquid solder when heat is applied to join the solder and the metal pad and thereby form the electrical connection. Other techniques use a solder preform which is placed between the substrate and device. Yet other techniques use solder bumps which are applied to the device or to the substrate.
Typically, soldering processes include three basic steps: (1) pre-cleaning and deoxidation of surface oxides; (2) solder reflow and/or reflow joining; and (3) post-soldering cleaning. Different flux materials are used in the pre-cleaning step to prepare the surfaces for the soldering step by removal of contaminants and metal oxides from the solder surface. For example, activated fluxes, such as zinc, ammonium chloride, mineral acid-containing materials, and the like, are typically used in "coarse" soldering applications, i.e., repairing coarse wiring in motors or houses. The solder joining step can occur only after the oxide coating is removed because the high melting point oxides will prevent wetting of the two surfaces to be joined by reflow of solder. The third step, post-soldering cleaning, removes flux residue remaining after the reflow.
Highly acidic fluxes are used for the soldering of aluminum layers. Aluminum has a tenacious oxide layer which is chemically very inert and difficult to remove. Thus, mild rosin fluxes are ineffective with aluminum, and special fluxes containing acid compounds which are highly corrosive, such as inorganic acids in a cadmium fluoroborate vehicle, must be used. Fluxes used with aluminum can also contain metal chlorides, fluorides, and ammonium compounds.
Because of the gross corrosive nature of these fluxes, and the high attack rates on metals in microelectronic assemblies, such fluxes cannot be used in microelectronics. For microelectronic devices, the standard practice is to reduce the acid activity of the flux to a mildly activated or non-activated grade in an attempt to minimize the adverse effects of the flux on the components. Typical soldering processes for copper layers in microelectronic applications use rosins which form a very mild organic acid when melted at the soldering temperature but which are relatively inert at room temperature.
Although corrosion and other risks can be minimized in copper soldering applications using mild flux agents, flux is necessary to keep the solder from oxidizing, allow it to flow and wet the parts being soldered. In addition, with the shrinking size of all electronic components and bonding pads, the rapidly growing use of surface mount technology, and the increasing demand for flip-chip device bonding, the post reflow cleaning of flux residues is becoming increasingly difficult. The small gaps between assembled parts, and solidification cavities in mixed soldered joints are very resistant to penetration by cleaning liquids. Inefficient post-soldering cleaning can reduce the long term reliability of the whole assembly. Further, there can be other problems associated with non-activated or mildly activated flux processes, such as higher defect levels and high rework costs. Optoelectronic devices are also very sensitive to flux residues due to absorption and bending of the optical signals.
A fluxless soldering process, particularly for soldering microelectronic devices including a copper layer, therefore can have several advantages. A dry or fluxless soldering process can replace the pre-cleaning step and totally eliminate the post-soldering cleaning step. Fluxless soldering has also gained increasing importance in recent years due to concerns for the environmental effect of common cleaning agents, such as chloroflurocarbons.
Various attempts at fluxless soldering have been made but with limited success. Some fluxless processes have used halogenated gases. For example, P. Moskowitz et al., J.Vac.Sci.Tech. 4, (May/June 1986) describe a dry soldering process for solder reflow and bonding of lead/tin solder. The process uses halogen-containing gases to reduce the surface oxide to enable solder reflow at temperatures above the solder-melting point. The activation energy needed for the oxide reduction by these gases is lowered by use of a catalyst, platinum mesh, in a vacuum chamber. Yet the temperature needed for successful reflow bonding is 300.degree. C., well above typical soldering temperatures for most electronic applications of about 220.degree. C. Thus, this process can damage the components, the substrate, and generate defects due to thermal mismatch between different materials.
IBM Technical Disclosure Bulletin 27 (April 1985) describes the use of halogenated gases in an inert carrier gas at elevated temperatures to produce a reduction of solder oxide by the reactive gas and to allow solder reflow. Again, for the more common low temperature applications, thermal damage may result.
P. Moskowitz et al., J.Vac. Sci.Tech. 3 (May/June 1985) describe a laser-assisted fluxless soldering technique for solder reflow. This technique uses laser radiation to excite an otherwise nonreactive gas in the presence of preheated solder surface. This technique requires direct access of the laser beam to the solder surface, thus limiting the applications as well as resulting in a low throughput process.
U.S. Pat. No. 4,921,157 discloses a fluxless soldering process for semiconductor devices. In the process, solder surface oxides are removed using a plasma process. Solder having a surface oxide layer is deposited onto a surface and fluorine-containing plasma excitation is performed on the solder. The solder is then reflowed.
German Patent No. 3,442,538 discloses a method of soldering semiconductor elements wherein a semiconductor element having an aluminum layer is subjected to a fluorine-containing plasma. The treated aluminum surface is then contacted with a soft solder. Process conditions include treating the aluminum layer with a fluorine-containing plasma for at least 1 hour in a vacuum at a temperature of about 147.degree. C. to 397.degree. C. Alternative process conditions use a standard soldering iron, presumably in the presence of flux, to remove oxides. Further, as with several of the processes described above, the temperatures used are well above typical soldering temperatures for most electronic applications, and can result in damage to the components.
The types of fluxes and flux conditions used for aluminum are very different than those used for copper soldering. Because of the nature of the tarnish finish of copper, mild rosin fluxes can be used. Copper forms only a mild galvanic cell with solder due to their close electromotive potentials (0.13 vs. -0.34 for tin and copper, respectively). Thus the corrosion risk for soldered copper is very low when mild rosin fluxes are used. Further, the attack of the copper and solder and other fine metal features of the microelectronic circuit is low enough to be acceptable in most soldering processes. See H. Manko, Solders and Soldering (McGraw Hill New York 1992), pp. 380-381; 156-158.
In contrast, aluminum has a tenacious surface oxide layer which is difficult to remove and which is chemically very inert. Special fluxes are used for aluminum which contain highly corrosive acid compounds, such as inorganic acids in a cadmium fluoroborate vehicle. Fluxes used with aluminum can also contain metal chlorides, fluorides, and ammonium compounds. The flux mechanism usually involves aluminum attack, forming aluminum chlorides which are gaseous at the soldering temperature and help disperse the oxide layer of the aluminum surface. Because of the gross corrosive nature of the fluxes, and the high attack rate on metals in microelectronic assemblies, these fluxes cannot be used for copper or in microelectronic applications. The particular combination of lead tin solder and aluminum is also very bad from a galvanic standpoint in that the potential difference between the solder and the aluminum (1.53 v) exceeds the tolerable range and fast deterioration of the joint occurs under humid conditions. See H. Manko, Solders and Soldering (McGraw Hill New York 1992), pp. 373-375.
Thus fluxes and conditions for the soldering of aluminum are very different than for the soldering of copper. H. Manko, Solders and Soldering (McGraw Hill New York 1992), pp. 160-161. What works for one will not work for the other, and vice versa. This is also true of controlled atmosphere soldering which makes use of reducing gases such as hydrogen or carbon monoxide, or organic acid gases such as formic acid or acetic acids. These have been demonstrated to have some applicability to copper but are woefully inadequate on aluminum. Thus it is not expected that a technique to solder aluminum would work with copper. In fact, one skilled in the art would expect just the opposite. See C. Mackay, Flux Reactions and Solderability in Solder Joint Reliability, J. Lau Editor (Van Nostrand Reinhold, New York, 1991), pp. 73-80.